Detergent compositions intended for mechanical dishwashing will usually require a component capable of providing a considerable alkalinity in the wash liquor. Alkali metal silicates having a SiO.sub.2 /M.sub.2 O molar ratio from about 1.5:1, preferably from 1.8:1, to about 3.3:1, preferably to 2.2:1, are particularly suitable for this purpose and also they protect glassware from corrosion.
Fabric washing compositions also require the presence of alkali metal silicates and market pressures are driving such compositions towards the use of higher bulk density products. Usually such compositions will have a bulk density above 700 g/l, preferably above 850 g/l and more preferably up to 1.0 kg/l. Typically these compositions are prepared free of phosphates and this route will require the presence of another builder, e.g. zeolites. The latter component, when spray dried in a slurry containing silicates forms insoluble agglomerates and thus there is a need to post dose silicates of appropriate bulk density. Alkali metal silicates formed separately in powder form or into granules in a spray drying step have lower bulk densities e.g. ca 650 g/liter, have a tendency to cake and are dusty.
Alkali metal silicate in granular form can be formed by a compaction process using the procedures described in U.S. Pat. No. 3,875,282 (Stauffer Chemical Co) and U.S. Pat. No. 3,931,036 (Philadelphia Quartz Co). A bulk density of ca 900 g/l is obtainable but the particle shape is not optimum because it derives from a broken compressed slat; additionally fines are produced from the particle edges by interparticle collision during handling. In practice a maximum bulk density of only about 850 g/l can be achieved with a compaction process.